Amplifier system



June 6, 1933.

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INVENTOR Byoger 07. W/se ,9,3 ATroRNEYs Patented June 6, 1933 UNTED STATES PATENT OFFIQE ROGER IVI. WISE, 0F NEW YORK, N. Y., ASSIGNOR OF ONE-HALF TO E. T. CUNNINGHAM, 0F NEW YORK, N. Y.

AMPLIFIER SYSTEM Application led May 19,

This invention relates generally to amplifier systems of the electron relay type, and particularly to amplifier systems for repeating currents of audio frequency.

It is an object of this invention to devise. an audio frequency amplifying system which will minimize distortion due to low frequency oscillations or motor boating.

1t is a further object of this invention to devise an audio frequency amplifier system which will be provided with novel means for neutralizing the feed back coupling` due to the resistance of a common source of plate potential.

Further objects of this invention will appear from the following description in which I have set forth the preferred embodiment of my invention. It is to be understood that the appended claims are to be accorded a range of equivalents consistent with the state of the prior art.

The single figure of the drawing is a circuit diagram illustrating the system of this invention.

Multi-stage audio frequency amplifiers, such as are used on radio receiving sets, coinmonly utilize a single source of potential for the plate circuits of the different amplifier stages. This source of potential usually consists of the ordinary dry cell B battery or of a B battery eliminator, that is, a properly filtered source of potential inherently including considerable resistance which is in common to all of the amplifier stages. For this reason a heavy surge of plate current in the last amplifier stage may be repeated to a preceding amplifier', and if in any particular preceding amplifier the fed back disturbance happens to be in phase with the original disturbance, the disturbance is reinforced and motor boating may result. By motor boating I refer to low frequency oscillations, sayV in the neighborhood of ten cycles which modulate the frequencies which it is desired to amplify, `and thereby cause distortion. The feedingl back of the ordinary amplified impulses occurring at frequencies within the voice range mayrordinarily be prevented by using a relatively large ley-pass condenser across the source o-f plate poten- 1927. Serial No. 192,552.

tial. However such a by-pass condenser is ineiective to by-pass relatively low frequencies since the reactance of the condenser for such frequencies is relatively high compared to the resistance value of the source i of plate potential. In this invention by-pass condensers are employed to prevent feeding back of impulses occurring at frequencies lying within the ordinary voice range, but additional means is provided in order to prevent positive feed back of energy to a preceding amplifier for frequencies which are so lou7 that the by-pass condensers are ineffective.

In the drawino' l have shown an electron 65 relay system comprising a plurality of relays 10 to 13 inclusive which are resistively coupled together in cascade. Relay 10 in this case performs the function of a detector or integrator While relays 11, 12 and 13 amplify the detected energy. When used in a radio receiving system, the input of detector 10 may be coupled tothe output of a suitable radio frequency amplifier Y1li, this amplifier having its inputcircuit coupled to suitable source of signal. energy such as provided by the antenna 15. These relays have been shown as being of the usual three element type having a grid 17, filament or electron emission element 18, and plate or anode 19. The input circuit or grid circuit of each of the amplifying relays 11, 12 and 13 are shown as coupled to the outputcircuit of a preceding relay by means of coupling resistances 20, 21, and 22. Suitable blocking condensers 30, 31 and 32 are preferably provided for the grid circuits of amplifier relays 11, 12 and 13, and the respective grids of these relays are also provided with suitable grid leak resistances 41, l2 and 43 respectively. As the last relay 13 is usually a power amplifier, a suitable C battery 26 may be provided to secure the proper negative grid bias.

The source of plate potential for the system has been represented by the B battery 2'?, and theinherent resistance of this battery been represented by the series resistance 28. The filaments or electron emission elements of the relays have been shown as energized from a suitable source of current 100 such as the A battery 29. The output circuit of the last amplifier relay 13 supplies a loud speaker or other translator 35, and instead of connecting this loud speaker directly in the output circuit, I preferably connect the same in series with a blocking condenser 36, and complete the plate circuit of tube 13 through the relatively large inductive choke 37. For by-passing the majority of frequencies lying Within the voice range, a relatively large reactance in the form of a condenser 38 is shown as shunted across the B battery 27 and its inherent resistance 28.

lilith the system as described above, substantially no objectionable feed back of energy Will occur due to the resistance 28, for the frequencies occupying the major portion of the voice range. However for very loiv audio frequencies, say below fifty to one hundred cycles, the reactance values of blocking condensers 30, 31 and 32, and of by-pass condenser 38 become` relatively high. As a result assuming that the resistors 20 to 22 were all connected directly to the positive terminal of batte-ry 27 represented by the conductor 39, potential variations across the source of plate potential would be fed back into certain of the preceding relay stages, and in case of positive feed back of energy, Would cause motor boating or cause oscillations at a lou' audio frequency. This may be explained by the fact that low frequency components of currents in the plate circuit of relay 13 will pass through choke 37 and the resistance 28 of the B battery. Thus low frequency voltage variations are set up across the B battery 27 which are impressed upon the plate circuits of relays 10, 11 and 12, and thus impressed upon the grids of relays 11 and 12. For audio frequency variations of high frequency the impulses fed back into the input circuits of relays 11 and 12 are negative or in a. direction opposite to the effect of the disturbing impulses upon the grids of these relays. However' for the low frequency voltage variations, the blocking condensers 30, 31 and 32 introduce aphase difference because of their rela-tively high reactance for these frequencies, and this phase difference may result inA positive rather than negative feed back to the relay 11. Feed back to relay 12 is positive for such low frequencies except to the extent that blocking condensers 31 and 32 are effective in shifting the phase.

In this in-vention in order to prevent motor boating, I provide means for neutralizing positive feed back of energy into the input of relay 11. This means is preferably in the form of a resistor 116 which provides a common path of resistance for connecting the terminals of resistors 20 and 21 to the conductor 39y or to the positive terminal of the B battery. A relatively large by-pass condenser 47 is also provided for by-passing the major portion of the voice frequency components, and this condenser is connected to the point of connection of resistor 46 with the resistors 20 and 21, and to the filament circuit. In other Words condenser 17 is connected in shunt with the source of plate potential, in series with the resistor 46. Bypass condenser 47 together with by-pass condenser 38 are e'ective to by-pass the higher audio frequency components so that for these frequencies substantially no feed back of energy occurs. However for the lower frequencies the rcactances of condensers 17 and 38 become so high that they are ineffective to by-pass the voltage components. Such low frequency components are repeated through relay 11 and passed through the common resistor L16, whereby they are repeated into the input circuit of relay 11 through the resistor 20. However the voltage variation impressed upon the grid of relay 11 by action of the common resistor 46, Will be 180 degrees out of phase with the original disturbing impulse upon the grid of relay 11. In other words the tendency of relay 11 to receive positive feed back of energy and thus to initiate motor boating, is offset by impressingupon its grid a neutralizing potential.

lVith an amplifying system as described above, the resistance of the source of plate potential has been experimentally increased to values in the neighborhood of 100.000 ohms Without causing motor boating. The system also operates satisfactorily from standard types of B battery eliminators. The neutralizing arrangement is most effec tive when applied to relay 11 as described, or stated more broadly, when applied to the odd numbered relays counting from the last stage.

I claim 1. In an electron relay system, a plurality of electron relays each having grid and plate circuits, means for resistively coupling together said circuits in cascade, a blocking condenser inserted in each grid circuit which is resistively coupled to a preceding relay, a. source of plate potential in common to said relays, said source having an appreciable resistance tending to form ay coupling between said relays for repeated currents of audio frequency, a Vreactance for by-passing a major portion of audio frequency components about the resistance of said source, and means for preventing positive feed back of energy to at least one of the relays caused by components ofy frequencies so low as to cause the blocking condenser to introduce a phase displacement and the resistance to cause intercoupling between the relays, said means comprising a resistor in common to the plate circuit of one relay to-Which positive feed back of energy has a tendency to occur, and to the plate circuit of the next preceding relay, and a. by-pass condenser shunted across said source of potential, one terminal of said condenser being connected tothe point of connechij tion of said common resistor to said last named plate circuits, said common resistor serving to impose a neutralized component upon the grid of said one relay for said low i frequency components for which the by-pass condenser is ineffective. 2. In an electron relay system, a plurality of electron relays each having grid and plate circuits, means for resistively coupling together said circuits in cascade, a blocking condenser inserted in each grid circuit which is resistively coupled to a preceding relay, a source of plate potential in common to said relays, said source having an appreciable resistance tending to form a coupling between said relays for repeated currents of audio frequency, a by-pass condenser shunted across said source for by-passing a major portion of audio frequency components about said source, and means for preventing positive feed back of energy to the third from the last relay, caused by components of fre uencies so low as to cause the blocking con ensers to introduce a phase displacement and the resistance of the source to cause positive feed back of energy to said third relay, said means comprising a resistor in common to the plate circuit oit the third from the last relay and to the plate circuit of the next preceding relay, and another by-pass condenser shunted across said source of potential, one terminal of said condenser being connected to the point of connection of said common resistor to said last named plate circuit, said common resistor serving to impose a neutralizing component upon the grid of said third relay for said loW frequency components for which the by-pass condensers are inetl'ective.

In testimony whereof, I have hereunto set 4o nu handE y ROGER M. WISE. 

